Photographic elements containing dithiolium salts

ABSTRACT

PHOTOGRAPHIC ELEMENTS HAVING A SUPPORT COATED WITH A LIGHT-SENSITIVE LAYER INCLUDING A BLEACHING AGENT AND A DITHIOLIUM SALT WHICH, UPON EXPOSURE. THE SULFUR ATOMS OF THE DITHIOLIUM SALT NUCLEUS CAN HAVE EITHER A 1,2- OR A 1,3-RELATIONSHIP, AND ALL CARBON ATOMS IN THE NUCLEUS CAN BE SUBSTITUTED WITH SUCH GROUPS AS HYDROGEN ATOMS, ALKYL RADICALS, AMINO RADICALS, MONO- OR POLYCYCLIC ARYL RADICALS AND OXYGEN- OR SULFUR-CONTAINING HETEROCYCLIC RADICALS. ADDITIONALLY, THE DITHIOLIUM NUCLEUS CAN FORM PART OF A FUSED RING AROMATIC STRUCTURE IN WHICH ANY OR ALL OF THE RINGS CAN BE SUBSTITUTED WITH ANY OF THE ABOVE-NOTED SUBSTITUENTS. THE BLEACHING AGENT IS A COMPOUND HAVING A THIOCARBONYL GROUP SUCH AS A THIOUREA COMPOUND.

United States Patent 3,574,624 PHOTOGRAPIHC ELEMENTS CONTAINING DI'IHIOLIUM SALTS George A. Reynolds and James A. Van Allan, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.Y. No Drawing. Filed Feb. 8, 1968, Ser. No. 703,895 Int. Cl. G03c 1/72 US. Cl. 96--89 18 Claims ABSTRACT OF THE DISCLOSURE Photographic elements having a support coated with a light-sensitive layer including a bleaching agent and a dithiolium salt which, upon exposure to activating radiation, is bleached to a colorless form, provide positive photographic images directly upon exposure. The sulfur atoms of the dithiolium salt nucleus can have either a 1,2- or a 1,3-relationship, and all carbon atoms in the nucleus can be substituted with such groups as hydrogen atoms, alkyl radicals, amino radicals, monoor polycyclic aryl radicals and oxygenor sulfur-containing heterocyclic radicals. Additionally, the dithiolium nucleus can form part of a fused ring aromatic structure in which any or all of the rings can be substituted with any of the above-noted substituents. The bleaching agent is a compound having a thiocarbonyl group such as a thiourea compound.

This invention relates to photography and more particularly to the preparation of photographic images by image-wise bleaching of a colored compound.

The production of photographic images by the use of conventionally designated bleachout systems has been extensively described. Particularly, a colored compound or composition is rendered colorless in an image-wise fashion upon exposure to activating electromagnetic radiation and without further processing. The selection of colored compounds for such bleachout systems has, however, been rather limited.

Accordingly, it is an object of the present invention to provide novel light-sensitive compositions that can be rendered colorless upon exposure to activating light radiation.

It is another object of this invention to provide new photographic elements for bleachout systems.

Other objects and advantages of the invention described herein will become increasingly apparent from a reading of the following specification and appended claims.

The objects of this invention are accomplished with light-sensitive, light-bleachable compositions including a colored dithiolium salt and, as a bleaching agent for said salt, a compound having a thiocarbonyl group. In the presence of the bleaching agent, a wide variety of colored 1,2- and 1,3-dithiolium salts are rendered colorless upon exposure to activating light radiation. Such light-bleachable compositions can be coated on a support material to produce photographic elements that, directly upon an imagewise light exposure, provide positive, colored photographic images.

The dithiolium salts useful in the present invention all have in common a dithiolium nucleus which comprises a five-membered heterocyclic ring which is generally aromatic in nature and which contains two sulfur atoms. The two sulfur atoms can be in either a 1,2- or a 1,3- relationship to one another. In addition, the dithiolium nucleus can be substituted at all carbon atoms with such substituents as hydrogen, alkyl radicals of from 1 to 5 carbon atoms, substituted amino radicals including ones containing from 1 to carbon atoms, aryl radicals including phenyl and naphthyl as 'well as substituted phenyl 3,574,624 Patented Apr. 13, 1971 and naphthyl radicals and heterocyclic radicals having at least one hetero atom selected from the group of oxygen and sulfur and containing up to 9 carbon atoms. Furthermore, the dithiolium nucleus can form part of a fused ring aromatic structure, such as benzodithiolium, with either or both rings being substituted with any of the substituents referred to above. Other dithiolium compounds also useful in this invention include those having two fused ring structures, such as benzodithiolium, linked together by an unsaturated aliphatic radical having from 1 to 5 carbon atoms.

The 1,2-dithiolium salts of this invention include such salts having a fused saturated fiveor six-membered ring in the 3,4-position, which fused ring can also be substituted with aralkenyl radicals including substituted aralkenyl radicals having on the aromatic portion such substituents as another aralkenyl radical; alkyl, alkoxy, alkylalkoxy, alkylarnino, alkylthio etc. radicals of from 1 to 10 carbon atoms; nitro radicals; hydroxy radicals; alkylenedioxy radicals and the like. In addition, the 1,2-dithiolium salts of this invention also include 3-substituted- 1,2-dithiolium salts wherein the 3-substituent can be an aryl radical such as phenyl, naphthyl etc. including substituted aryl radicals having a hydroxy, alkoxy, alkylenedioxy or tertiaryamino radical thereon; an aralkenyl radical having 2 to 4 carbon atoms in the alkenyl moiety and including substituted aralkenyl; and an aromatic heterocyclic alkenyl radical wherein oxygen is the hetero atom and including the substituted radicals which can contain, among others, aralkenyl groups as mentioned above.

The 1,3-dithiolium salts useful in the present invention include 1,3-benzodithiolium salts and 4,5-dihydro- 1,3-dithiolium salts including the corresponding Z-substituted salts wherein the 2-substituent can be an amino radical including alkyl and arylsubstituted amino radicals; an alkyl radical of from 1 to 12 carbon atoms including substituted alkyl radicals; an aryl radical including substituted aryl radicals having such substituents as alkyl groups of from 1 to 5 carbon atoms, alkoxy groups of from 1 to 5 carbon atoms, alkylamino groups of from 2 to 10 carbon atoms etc.; an aralkenyl radical wherein the alkenyl moiety contains from 2 to 4 carbon atoms and including substituted aralkenyl in which the aromatic portion has such substituents as alkyl, alkoxy, alkylamino, alkylthio etc. radicals all of from 1 to 10 carbon atoms, nitro radicals, hydroxy radicals, alkylenedioxy radicals etc.; and a heterocyclic alkenyl radical having at least one oxygen or sulfur hetero atom and including such radicals substituted on the heterocyclic moiety with any of the substituents included with the aralkenyl radicals above as well as heterocyclic alkenyl radicals substituted with aralkenyl radicals such as those listed above.

Included among the many types of useful 1,2- and 1,3- dithiolium salts are the following general categories:

(a) 5-aryl-1,2-dithiolium salts substituted at the 3-position with an alkyl radical of from 1 to 5 carbon atoms, an aryl radical including substituted aryl radicals containing such substituents as hydroxy, alkoxy or tertiaryamino radicals, an aralkenyl radical in which the aryl moiety can be phenyl, naphthyl or an aromatic heterocyclic radical wherein the hetero atom is oxygen and having from 2 to 4 carbon atoms in the alkenyl moiety, and wherein the 5- aryl radical can be substituted with hydroxy, alkoxy, alkylenedioxy, alkylarnino etc. radicals and halogens;

(b) 3-alkylthio-l,Z-dithiolium salts containing a benzylidene substituted fused saturated ring in the 4,5-position wherein the fused ring can be a fiveor six-membered ring and the benzylidene radical can be substituted with lower alkoxy, alkylamino, styryl etc. radicals as well as with a methylidyne-3-alkylthio-1,2-dithioliurn salt radical containing a fused ring at the 4,5 position, and wherein the 3- alkylthio radical contains from 1 to 5 carbon atoms;

(c) 2-amino-4-aryl-1,3-dithiolium salts wherein the amino and aryl groups can be substiuted with aryl or alkyl radicals;

(d) 2 substituted -alkyl-1,3-benzodithiolium salts wherein the 2-substituent can be an alkyl radical of 1 to 5 carbon atoms, an aryl radical including substituted aryl having substituents as lower alkyl radicals and lower alkoxy radicals of from 2 to carbon atoms etc., an aralkenyl radical having 2 to 4 carbon atoms in the alkenyl moiety, an oxygenor sulfur-containing heterocyclic alkenyl radical having from 2 to 4 carbon atoms in the alkenyl moiety and a vinyl radical including 2-substituted vinyl radicals having such substituents as an aryl or an aromatic heterocyclic radical including aryl and heterocyclic radicals having such substituents as lower alkoxy, alkylamino, nitro, alkylenedioxy, styryl etc. radicals, and wherein the S-alkyl radical has from 1 to 5 carbon atoms;

(e) 2 aminoaryl 4-aryl-1,3-dithi0lium salts wherein either aryl moiety may be substituted and wherein the nitrogen of the amino group can be substituted with alkyl, hydroxyalkyl or aryl radicals;

(f) 4,5-dihydro1,3-dithiolium salts substituted at the 2-position with aryl radicals, styryl radicals, including substituted aryl and styryl radicals, and oxygenor sulfurcontaining heterocyclic alkenyl radicals;

(g) 2-aralkeny1-4-aryl-1,3-dithiolium salts wherein the 2-aralkenyl radical is as the aralkenyl radical referred to in (a) above and wherein the 4-aryl radical is the same as the S-aryl in (a) above; and

(h) Bis-(S-alkyl-1,3-benzodithiolium) alkene salts.

The acid anions associated with the dithiolium cations to form salts include such anions as perchlorate, chloride, bisulfate, methyl sulfate etc. Useful 1,2- and 1,3-dithiolium salts thus include, among others, salts of the cations listed in the table below:

TABLE I l) 3-(2-hydroxy-1-naphthyl) -5-phenyl-l,2-dithiolium (2) 3-(4-hydroxy-l-naphthy1)-5-phenyl-1,2-dithiolium (3) 3-(2,4-dihydroxypl1enyl)-5-phenyl-1,2-dithiolium (4) 3- [4-(N-2-hydroxyethyl-N-methylamino -2-ethylphenyl] -5-phenyl-1,2-dithiolium 5) 3-(2,4-diethoxyphenyl) -5-methylphenyl-1,2-dithiolium (6) 3-(4-diethylamino-Z-hydroxyphenyl)-5-phenyl-1,2-

dithiolium (7) 3-(4-hydroxy-3-ethoxypheny1)-5-phenyl-1,2-

dithiolium (8) 3-(2,3,4-trihydroxyphenyl) -5-phenyl- 1,2-dithiolium (9) 3-(2,4-dibutoxyphenyl)-5-phenyl-1,2-dithi0lium 10) 3- 2,4-diethoxystyryl) -5-phenyl-1,2-dithiolium 11 3 [2-(5-styryl-2-furyl)vinyl] -5-phenyl-1,2-

dithiolium 12) 3-(Z-methoxystyryl)-5-phenyl-1,2-dithiolium l3 3- (4-ethoxystyryl -5-pheny1- 1,2-dithiolium (14) 3-(4-diethylaminostyryl) 5-phenyl- 1,2-dithiolium 15) 3-(2,3-diethoxystyryl)-5-pheny1-1,2dithiolium 16) 3- (2,3-dihydroxystyryl) -5-phenyl-1,2-dithiolium (17 3 (4-chlorostyryl) -5-phenyl-1,2-dithiolium (18) 3- 3,4-methylenedioxystyryl) -5-phenyl-1,2-

dithiolium (19) 3-(4-dimethylaminostyryl)-5-phenyl-1,2-dithio1ium (20) 6-p-diethylaminobenzylidene-3-methylthio-cyclopent[c] -1,2-dithiolium (21) 7-benzylidene-3-ethylthio-cyclohex [0] -1,2-

dithiolium (22) 7- 3,4-diethoxybenzylidene-3-methylthio-cyclohex [c]-1,2-dithio1ium (23 2-piperidino-4-phenyl-1,3-dithiolium (24) 2-diethylamino-4-p-toly1-1,3-dithioliunr (25 2-diethylamino-4-phenyl-1,3-dithio1ium (26) 2-[bis(p-ethoxypheuyl)vinyl]-5-methyl-1,3-

benzodithiolium (27) 2-(2,4-diethoxystyryl)-5-ethyl-1,3-benzodithiolium (28) 2[2-(5-styryl-2-furyl)vinyl]-5-ethy1-1,3-benzodithiolium (29) 2-p-tolylvinyl-5-ethyl-1,3-benzodithiolium (30) Z-furylvinyl-S-methyl-1,3-benzodithiolium (3 1 2-phenyl-5-ethyl-1,3-benzodithiolium (32) 2-p-anisyl-5-methyl-1,3-benzodithiolium 3 3 2-propoxyphenyl-S-methyl-1,3-benzodithiolium (34) 5-propyl-2-p-tolyl-1,3-benzodithiolium (3 5 2-m-dimethylaminophenyl-S-methyl-1,3-benz0- dithiolium (36) S-methyl-Z-phenyl-1,3-benzodithiolium (37) 2-styryl-5-propyl-1,3-benzodithiolium (38) 2(p-ethoxystyryl)-5-ethyl-1,3-benzodithiolium (39) 2-(3,4-diethoxystyryl) -5-methyl-1,3-benzodithiolium (40) 2-(4-hydroxy-3-ethoxystyryl)-5-methyl-1,3-

benzodithiolium (41) 2-p-dipropylaminostyryl-S-methyl-1,3-benzodithiolium (42) 2-p-diethylaminostyryl-S-methyl-1,3-benzodithiolium (43) 5-ethy1-2-p-nitrostyryl-1,3-benzodithiolium (44) S-methyl-Z- 3,4-methylenedioxystyryl -1,3-

benzodithiolium (45 2- (Z-butoxystyryl) -5-methyl-1,3-benzodithiolium (46) 2- 1-naphthylvinyl)-5 -ethyl-1,3-benzodithiolium (47) 2-(4-styrylstyryl)-5-methyl-1,3-benzodithiolium (48) 2 [bis (4-diethylaminophenyl vinyl]-5-methyl-1,3-

benzodithiolium (49) 2-(4-diethylaminostyry1)-4-pheny1-1,3-dithiolium (50) 2-(4-ethoxystyryl)-4-phenyl-1,3-dithiolium (5 1 2- 4-diphenylaminophenyl) -4-phenyl-1,3-

dithiolium:

(52) 2-(4-diethylaminophenyl)-4-phenyl-1,3-dithiolium (5 3 Z-{P- [N-propyl-N- (2-hydroxymethyl amino] phenyl}-4-phenyl-l,3-dithiolium (54) 2-(2,4-diethoxystyryl)-4,5-dihydro-1,3-dithiolium (55) 2- (4-ethoxystyryl) -4,5-dihydro-1,3-dithiolium (56) 2-(l-naphthylvinyl)-4,5-dihydro1,3-dithiolium (57) 2-styryl-4,5-dihydro-1,3-dithiolium (58) 2-[2-(S-styryl-Z-furyl)vinyl]-4,5-dihydro-1,3-

dithiolium (59) 2-(4-dipropylaminostyryl)-4,5-dihydro-l,3-

dithiolium (60) 2- 3- (4,5-dihydro-1,3-dithiole-2-yl)-1-propenylidene]-4,5-dihydro-1,3-dithiolium (61) 3-(9-julolidyl)-5-phenyl-1,2-dithiolium (62) 2-p-dimethylaminophenyl-S-methyl-1,3-benzodithiolium As noted herein, the subject dithiolium salts are colored compounds which, in the presence of a bleaching agent are bleached to a colorless form upon exposure to light which is typically high energy radiation like ultraviolet light. As such, the combination of at least one bleaching agent and at least one of the subject dithiolium salts forms a light-sensitive composition that, directly upon an imagewise light exposure, is rendered colorless in an exposed imagewise pattern and thereby produces a positive, colored photographic image.

Bleaching agents that are advantageous in the practice of the present invention include compounds having a thiocarbonyl group such as thiourea compounds. Typical such thioureas include those having the formula:

wherein R can be either a hydrogen atom, an alkyl radical or an alkenyl radical, which substituent radicals can be additionally substituted. Exemplary of bleaching agents useful herein are compounds such as:

thiourea,

1-a1lyl-2-thiourea,

s-Diethylthiourea,

N-allyl-N'-(fi-hydroxyethyDthiourea, and Diallylthiourea J To prepare the light-sensitive compositions described herein, at least one of the colored dithiolium salts of the present invention and a sensitizing or bleach-promoting amount of at least one of the subject bleaching agents are merely admixed, typically in a common solvent such as acetonitrile. Other advantageous solvents include organic solvents like acetone, methoxyethanol, ethoxyethanol, methanol, ethylacetate, toluene, xylene, trichloroethylene, methylenechloride etc., and various mixtures of solvents such as those mentioned herein. Generally, any solvent can be used that will dissolve the image-forming compounds, i.e. the colored dithiolium salt and bleaching agent, and the binder when a binder is used. The proportions of dithiolium salt and bleaching agent are subject to wide variation, and although only a bleach-promoting amount of bleaching agent is required, weight ratios of from about 1:4 to about 4:1 are conventionally employed. Ordinarily, a weight ratio of from about 3 :4 to about 4:3 is preferred.

Coating the image-forming combination of bleaching agent and dithiolium salt upon a support material forms a composite photographic element whereupon an imagewise light exposure produces positive colored photographic images. One or more light-sensitive layers can be coated in composite element. Typical support materials include polymers such as esterified cellulose derivatives like cellulose acetate, cellulose nitrate and cellulose acetate butyrate as well as other polymers such as polystyrene and poly(ethylene terephthalate). Papers, including polyethylene-coated and polypropylene-coated paper, are also advantageous support materials. The method of coating can be any of those known in the art and include such means as flow coating, dipping, swabbing and other more precise coating techniques such as doctor blade and hopper coating. In the coating solution, solid content is dependent upon particular coating conditions and typically varies from about 1% by weight to about by weight. Coating coverages are widely variable, and need only be sufficient to produce a colored layer of such a suitable density as will permit adequate image definition upon exposure and concomitant bleaching in exposed areas.

In addition to the noted active ingredients, binders, coating aids and the like addenda can be included in the coating solution where desired. Typical binders used to advantage herein include any film-forming material which is soluble in the solvents that dissolve the photoactive-forming compounds. Included among these binders are cellulose esters, e.g., ethyl-cellulose phthalate, cellulose acetate hydrogen phthalate, cellulose acetate with various amounts of free hydroxyl groups, etc.; phthalated polyvinyl acetals with varying amounts of phthalation; synthetic elastomers, e.g., polyvinyl toluene, styrene butadiene resin, copolymers of ethylacrylate and acrylic acid, etc.

The preferred binders are water-insoluble waterpermeable materials such as cellulose acetate which is from about 20 to about 35% acetylated, and cellulose acetate hydrogen phthalate.

Subsequent to coating and drying the composite photographic elements so prepared are susceptible of recording colored photographic images.

The production of a photographic image on the subject elements is accomplished merely by an imagewise light exposure. Suitable light radiation includes that of a tungsten light source, as well as ultraviolet and actinic light radiation. After a suitable exposure and in the presence of at least one of the bleaching agents described herein, the colored dithiolium salt is reduced to a colorless form in the areas of exposure. The length of exposure is variable and depends on the intensity of the exposing source and its distance from the light-sensitive element. One can readily determine by visual means when sufiicient bleaching has taken place.

After the bleachout image has been formed by exposure, it can be stabilized against subsequent fading by treating the light-sensitive layer with water, preferably warm water.

The following examples serve to further illustrate the invention.

EXAMPLE 1 A solution of .l g. of 3-(9-julolidyl)-5-phenyl-l,2- dithiolium perchlorate and .1 g. of 1-allyl2-thiourea in 10 ml. of acetonitrile is flow coated onto a paper support material. After drying the coating has a blue color. The composite coated element is then exposed for 1 minute through a photographic negative to the light of a 500 Watt R-2 photoflood tungsten light held at a distance of about 1 foot from the exposure plane. At the end of the exposure, the areas of the coating impinged upon by the light are colorless and the unexposed areas remain blue. Washing with warm water stabilizes the bleachout image.

EXAMPLE 2 Photographic elements are prepared with the same preparation of dithiolium dye and thiourea compound as in Example 1, except that the colored dithiolium salts employed are perchlorate salts of the following cations:

(a) 2-p-dimethylaminophenyl-S-methyl-1,3-

benzodithiolium (b) 2-m-dimethylaminophenyl-S-methyl-1,3-

benzodithiolium (c) 3-(4-dimethylaminostyryl)-5-phenyl-1,2-

dithiolium (d) 2-[2-(5-styryl-2-furyl)vinyl]-4,5-dihydro-1,3-

dithiolium (e) 3-(2-hydroxy-l-naphthyl)-5-phenyl-1,2-dithiolium (f) 3-(2,3,4-trihydroxyphenyl)-5-phenyl-1,2-dithiolium (g) 6-p-diethylaminobenzylidene-3-methylthio-cyclopent- [c]-1,2-dithiolium and the bleaching agent is N-allyl-N-(B-hydroxyethyl) thiourea. The composite elements are then exposed as in Example 1. After exposure, the light-struck areas are substantially colorless, and the unexposed areas maintain their original color. Stabilization is accomplished with a Water wash.

The invention has been described in considerable detail with reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A light-sensitive, light-bleachable composition comprising a colored dithiolium salt and, as a bleaching agent for said salt, a bleach-promoting amount of a thiourea compound having the formula:

wherein R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical and an alkenyl radical.

2. A composition as described in claim 1 wherein the dithiolium salt is selected from the group consisting of a 5-aryl-1,2-dithiolium salt, a 4-aryl-1,3-dithiolium salt, a 5-alkyl-1,3-benzodithiolium salt and a 4,5-dihydro-l,3- dithiolium salt.

3. A composition as described in claim 1 wherein the dithiolium salt is a 5-aryl-1,2-dithiolium salt substituted at the 3-position with a radical selected from the group consisting of an alkyl radical having from 1 to 5 carbon atoms, an aryl radical and an aralkenyl radical having from 2 to 4 carbon atoms in the alkenyl moiety.

4. A composition as described in claim 1 wherein the dithiolium salt is a 4-aryl-l,3-dithiolium salt substituted at the 2-position with a radical selected from the group consisting of an amino radical, an aryl radical, an aralkenyl radical and a heterocyclic alkenyl radical.

5. A composition as described in claim 1 wherein the dithiolium salt is a 5-a1kyl-1,3-benzodithiolium salt substituted at the 2-position with a radical selected from the group consisting of an alkyl radical having from 1 to 5 carbon atoms, an aryl radical, an aralkenyl radical having from 2 to 4 carbon atoms in the alkenyl moiety and a vinyl radical.

6. A composition as described in claim 1 wherein the dithiolium salt is a 4,5-dihydro-1,3-dithiolium salt substituted at the 2-position with a radical selected from the group consisting of an aryl radical, an aralkenyl radical having from 2 to 4 carbon atoms in the alkenyl moiety and a heterocyclic alkenyl radical having from 2 to 4 carbon atoms in the alkenyl moiety.

7. A composition as described in claim 1 wherein the dithiolium salt is a bis-(5-alkyl-1,3-benzodithiolium)alkene salt wherein the alkyl and the alkene moieties each have from 1 to 5 carbon atoms.

8. A composition as described in claim 1 wherein the dithiolium salt is 5-phenyl-3-(9-julolidyl)-1,2-dithiolium perchlorate.

9. A composition as described in claim 1 wherein the dithiolium salt is Z-(p-dimethylaminophenyl) -5-methyl- 1,3-benzodithio1ium perchlorate.

10. A light-sensitive, light-bleachable composition comprising a dithiolium salt selected from the group consisting of a colored 1,2-dithio1ium salt and a colored 1,3- dithiolium salt and, as a bleaching agent for said salt, a bleach-promoting amount of a thiourea compound having the formula:

11 s H Ra's-Elisa wherein R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical and an alkenyl radical.

11. A composition as described in claim 10 wherein the thiourea compound is selected from the group consisting of thiourea, 1-allyl-2-thiourea, s-diethylthiourea, N-allyl-N'-( 8hydroxyethyl)thiourea and diallylthiourea.

12. A composition as described in claim 10 wherein the thiourea is 1-allyl-2-thiourea.

13. A composition as described in claim 10 wherein the thiourea is N-allyl-N'-(fi-hydroxyethyDthiourea.

14. A light-sensitive, light-bleachable composition as described in claim 1 wherein the colored dithiolium salt and the thiourea compound are in a weight ratio of from about 1:4 to about 4: 1.

15. A photographic element comprising a support having coated thereon a layer comprising a light-sensitive, light-bleachable composition as described in claim 2.

16. A photographic element having coated thereon a layer comprising a light-sensitive, light-bleachable composition as described in claim 3.

17. A photographic element as described in claim 14 wherein the support comprises paper.

18. A process for producing positive, colored photographic images on a photographic element comprising a support having coated thereon a light-sensitive, lightbleachable layer comprising a colored dithiolium salt, and as a bleaching agent for said salt, a bleach-promoting amount of a thiourea compound having the formula:

H s H Ra's-(Lila wherein R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical and an alkenyl radical, which process comprises imagewise light exposing said photographic element to light for a period of time sufficient to render said colored dithiolium salt colorless in the exposed areas.

References Cited UNITED STATES PATENTS 1,955,898 4/1934 Wendt et a1. 9689 3,141,891 7/1964 Klingsberg 260-327C 3,155,682 11/1964 Klingsberg 260-327 Meta NORMAN G. TORCHIN, Primary 'Examiner R. FIGHTER, Assistant Examiner US. Cl. X.R.

260327 Meta, 327C 1 

